In order to open any DC circuit, the inductive energy stored in the magnetic fields due to the flowing current must be absorbed; it can either be stored in capacitors or dissipated in resistors (arcs that form during opening the circuit are in this sense a special case of a resistor). A great difficulty of using ohmic resistors to define the resistance levels for a commutating circuit breaker is that (1) the transient voltage increase for each resistance level depends on current flowing and resistance inserted at the time of the commutation, and (2) the rate of current increase (during the fault) or decay (after resistance insertion) depends mainly on the inductance in a “dead short” which is the most severe kind of fault, in which the system resistance goes to nearly zero; inductance and system resistance (outside the circuit breaker) can vary a lot in real faults. Therefore, it would be ideal to calculate and define the proper resistance levels to insert each time the circuit breaker operates to reach a target maximum transient voltage difference across the inserted resistor, but this is not practical using ohmic resistors.
When varistors, reversed Zener diodes, or transorbs are put into the circuit, they create an opposed electromotive force (EMF) that absorbs stored energy in a fault; this can be viewed as a highly non-linear resistance, but it is also reasonable to view it more like a battery that loses all the energy put into it during charging, but still manages to control the voltage of the “charging current” fairly well.
Because of the rapid inrush of current in a short circuit, the inductive energy can easily be much greater than just the inductive energy stored in the system at normal full load; if the current goes to five times the normal full load amps before being controlled, the inductive energy would be up to twenty-five times as large as in the circuit at normal full load (depending on the location of the short). Until recently, testing standards for DC breakers have assumed slow operation corresponding to arc chute circuit breakers (the standard DC breaker design since the time of Edison), where the time to open the electrodes is typically greater than or equal to three milliseconds (ms) after the trip signal is received; it can take even longer (up to ten ms) to reach the point at which current begins to decrease. This means that high currents can build up in a short circuit through an arc chute breaker, potentially reaching the maximum capability of the DC power source. For this reason, the DC breaker standard applied in the US to circuit breakers for electric trains (ANSI/IEEE 37.20) requires the circuit breaker to be able to handle 200,000 amperes (200 kiloamps, “kA”), about the maximum short circuit current on a DC fault in an electric train subway system.
A second kind of mechanically switched DC circuit breaker includes the innovative, fast acting high speed vacuum circuit breaker (HSVCB) DC circuit breakers from Hitachi (see for example U.S. Pat. No. 4,216,513) which are based on using inductors and capacitors to create an L-C resonant circuit, coupled with an AC vacuum circuit breaker to break the current as it passes through zero. These circuit breakers expose insulation and circuit components of the normally DC circuit to rapid voltage reversal and voltage spikes. A lower maximum current (50 kA) is allowed by the Japanese regulators (standard JEC-7152) for the L-C resonant circuit breakers for use on DC rail applications compared to the 200 kA that must be withstood by the slower arc chute electric rail breakers. This is enabled by the faster circuit opening action of the L-C based resonant breaker. Essentially in such a breaker, a capacitor discharge (which is triggered electronically) sets up an L-C resonant oscillation which causes the current to oscillate through zero, much like an AC circuit. This oscillation decays rapidly, but during the decay, a vacuum circuit breaker opens the circuit as the current crosses zero. A recent U.S. patent applicationb Ser. No. (13/697,204) shows that this mechanism can also be applied to high voltage DC (HVDC) circuits.
The fastest known way to switch off DC power is to use switchable power electronic devices to open the circuit; these are typically semiconductors, either thyristors or transistors, but vacuum tubes can also be used. In these designs, the resistance of the switch per se is an important consideration, as the full circuit load goes through the switch in the on-state. In the case of the most common type of power electronic switches, the integrated gate bipolar transistors (IGBTs), the typical on-state loss would be between 0.25-0.50% of transmitted power, which is unacceptably large for many applications, and also implies a significant cooling load for high power circuits, which typically requires a pumped liquid coolant. The need for active cooling increases cost and environmental impact, and decreases the reliability of the switch.
ABB has been the main developer of another method to speed up operation of DC switches, including circuit breakers, while maintaining lower on-state losses than purely power electronic circuit breakers, which is a hybrid of power electronic and mechanical switches. In this hybrid method, there are at least two power electronic switches combined with a fast mechanical switch. The first power electronic switch is a low-loss, low voltage-withstand switch that commutates the current to a second path through a second power electronic switch with high voltage withstand capability (but higher on-state losses). Said second power electronic switch may be comprised of a stack of IGBT transistors, a stack of gate turn off (GTO) thyristors, or various kinds of tubes which are capable of shutting off the current. Before said second power electronic switch can be turned off electronically, the low voltage withstand first electronic switch must be protected from the resultant voltage surge by a series-connected mechanical switch; the second high voltage capability shutoff switch cannot open the circuit until the moveable electrodes of the mechanical switch reaches the minimum separation to prevent striking or restriking an arc. This series-connected mechanical switch is the slowest component of the switch, so making it faster makes the hybrid switch faster. The currently used fast mechanical switches have electrodes that are magnetically accelerated via electromagnet repulsion or by a capacitor discharge through a Thompson coil (induced magnetic repulsion), and the electrodes separate in a vacuum or in a gas, which could be a sulfur hexafluoride gas or gaseous mixture.
In hybrid breakers for medium voltage DC (MVDC) said first low voltage withstand switch is desirably an IGCT (integrated gate commutated thyristor); for high voltage DC (HVDC) hybrid breakers, said first low voltage withstand switch is desirably a single stage IGBT that commutates the current over to an IGBT array, with many series-connected IGBTs, with each IGBT in parallel with a metal oxide varistor (MOV). The second high voltage capability shutoff switch can comprise a series connected IGBT transistor array, a stack of gate turn off thyristors (GTOs), a cold cathode vacuum tube, or a similar power electronic switch capable of shutting off the power flow.
There is reported to be about a 100 microsecond response delay time in a Thompson coil actuated mechanical switch due to mechanical response of the connected electrode.
If the hybrid switch is also a circuit breaker, there must also be an energy absorbing snubber such as a semiconductor blocking device or a capacitor bank (for example) to absorb the inductive energy stored in the magnetic fields created by the current. The hybrid breaker described above is an example in which most of the stored inductive energy in an HVDC circuit, which can be more than 100 megajoules, is absorbed by a semiconductor blocking device during operation of a circuit breaker.